In recent years, as various kinds of movable bodies, vehicles such as an electric motorcycle, an electric automobile, and the like have been electrified after development and practical realization of an electric bicycle, an electric cart, and the like. Specifically, while replacing an automobile that utilizes an internal combustion engine as its driving power source, a hybrid automobile, which utilizes a gasoline engine and an electric motor as its driving power source, an electric automobile, which utilizes, as its driving power source, an electric motor that operates with a domestic electric power source or a battery to be charged through an electric charger installed at a gas station or an electric power supply station, and a fuel cell automobile, which travels while generating electricity through a fuel cell that utilizes hydrogen gas or the like, as a fuel, have sequentially been developed; a hybrid automobile, an electric automobile, and the like have already been put to practical use and have started to spread.
In the case of each of a gasoline vehicle, a diesel vehicle, and a motorcycle (hereinafter, described as “a conventional automobile and the like”) that each utilize a conventional internal combustion engine as the driving power source therefor, an engine sound and an exhaust sound, which are emitted by the driving power source itself, and road noise or the like during its travel are generated; therefore, a pedestrian walking in a town, a cyclist, or the like can be aware of approach of a vehicle through an engine sound, an exhaust sound, or the like. However, because during a low-speed travel, a hybrid automobile travels mainly by means of not an engine but an electric motor, no engine sound or exhaust sound is generated, and an electric automobile, a fuel cell automobile, and the like each travel by means of an electric motor in the whole driving region; thus, any one of these automobiles is extremely high-quietness electric vehicle. In this regard, however, a pedestrian or a cyclist in the vicinity of such a high-quietness electric vehicle cannot perceive through a sound an approach of the electric vehicle such as a hybrid automobile, an electric automobile, or a fuel cell automobile that travels by means of a less-sound-noise and high-quietness electric motor; therefore, this may become the cause of a minor collision between the high-quietness electric vehicle and the pedestrian or the like.
Accordingly, in order to solve the foregoing problem in which the quietness, which is originally a merit of each of a hybrid automobile, a fuel cell automobile, and an electric automobile, provides an adverse effect from time to time, there have been proposed various kinds of approaching vehicle audible systems, other than a horn that is provided in a conventional automobile or the like and sounds an alarm in accordance with the will of a driver, that are to notify a pedestrian in the vicinity of a reference vehicle of the existence of the reference vehicle (for example, Patent Documents 1 and 2).
In an approaching vehicle audible system, a filter or the like is applied to a sound element utilized as a sound source, in accordance with an accelerator opening degree or a vehicle speed, so as to change a sound element signal and hence control the sound quality in order to obtain a natural notification sound.
Meanwhile, sound elements are prepared mainly by automobile manufacturers and differ depending on the kinds of automobiles. Therefore, it is required to adjust a filter or the like for each of various kinds of sound elements. Moreover, in the case where the sound quality that has been set by default by an automobile manufacturer is not preferable one for a driver, no device for simply adjusting the sound quality is prepared even though the driver wants to adjust it.